Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Non-volatile memory is memory that can retain its stored data for some extended period without the application of power. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones and removable memory modules, and the uses for non-volatile memory continue to expand.
Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming (which is sometimes referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data state of each cell. Data can be read from the memory cells by performing a read operation. Memory cells are typically programmed using erase and programming cycles. For example, memory cells of a particular block of memory cells are first erased and then selectively programmed.
Memory cells (e.g., flash memory cells) can be configured as what are known in the art as Single Level Memory Cells (SLC) or Multilevel Memory Cells (MLC). SLC and MLC memory cells assign a data state (e.g., as represented by one or more bits) to a specific range of threshold voltages (Vt) stored on the memory cells. SLC memory permits the storage of a single binary digit (e.g., bit) of data on each memory cell. Meanwhile, MLC technology permits the storage of two or more binary digits per cell, depending on the quantity of Vt ranges assigned to the cell and the stability of the assigned Vt ranges during the lifetime operation of the memory cell. The number of Vt ranges (e.g., levels) used to represent a bit pattern comprised of N-bits might be 2N, where N is an integer. For example, one bit may be represented by two ranges, two bits by four ranges, three bits by eight ranges, etc. MLC memory cells may store even or odd numbers of bits on each memory cell, and schemes providing for fractional bits are also known. A common naming convention is to refer to SLC memory as MLC (two level) memory as SLC memory utilizes two Vt ranges in order to store one bit of data as represented by a 0 or a 1, for example. MLC memory configured to store two bits of data can be represented by MLC (four level), three bits of data by MLC (eight level), etc.
FIG. 1 illustrates an example of Vt ranges 100 for a population of MLC (four level) (e.g., 2-bit) memory cells. For example, a memory cell might be programmed to a Vt that falls within one of four different Vt ranges 102-108 of 200 mV, each being used to represent a data state corresponding to a bit pattern comprised of two bits. Typically, a dead space 110 (e.g., sometimes referred to as a margin and might have a range of 200 mV to 400 mV) is maintained between each range 102-108 to keep the ranges from overlapping. As an example, if the Vt of a memory cell is within the first of the four Vt ranges 102, the cell in this case is storing a logical ‘11’ state and is typically considered the erased state of the cell. If the Vt is within the second of the four Vt ranges 104, the cell in this case is storing a logical ‘10’ state. A Vt in the third Vt range 106 of the four Vt ranges would indicate that the cell in this case is storing a logical ‘00’ state. Finally, a Vt residing in the fourth Vt range 108 indicates that a logical ‘01’ state is stored in the cell. For a memory cell having a particular data state represented by a bit pattern ‘XY’, the ‘X’ position bit might be considered the Most Significant Bit (MSB) and the ‘Y’ position bit might be considered the Least Significant Bit (LSB), for example.
Determining the data state of a selected memory cell involves performing a sense (e.g., read) operation on the memory cell. During the sense operation, a sense potential which increases over time can be applied to the selected memory cell. The MSB and the LSB of the data state of a selected memory cell can be determined when the applied sense potential has reached the highest level to be applied to the selected memory cells. However, waiting to determine both the MSB and LSB can result in a delay which can limit how fast data can be read from the memory device during a sense operation, for example.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present disclosure, there is a need in the art for alternate methods for performing data sensing operations in memory devices.